Opto electrical test measurement system for integrated photonic devices and circuits

ABSTRACT

An optical testing circuit on a wafer includes an optical input configured to receive an optical test signal and photodetectors configured to generate corresponding electrical signals in response to optical processing of the optical test signal through the optical testing circuit. The electrical signals are simultaneously sensed by a probe circuit and then processed. In one process, test data from the electrical signals is simultaneously generated at each step of a sweep in wavelength of the optical test signal and output in response to a step change. In another process, the electrical signals are sequentially selected and the sweep in wavelength of the optical test signal is performed for each selected electrical signal to generate the test data.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.15/489,127 filed Apr. 17, 2017, which is a continuation-in-part of U.S.patent application Ser. No. 15/133,614 filed Apr. 20, 2016, now U.S.Pat. No. 9,791,346, the disclosures of which are incorporated byreference.

TECHNICAL FIELD

The present disclosure relates to the field of photonics, and, moreparticularly, to optical measurement of photonic devices and circuits.

BACKGROUND

Integrated optical devices (for example, photonic integrated circuits)for directly processing optical signals have become of greaterimportance as optical fiber communications increasingly replace metalliccable and microwave transmission links. Integrated optical devices canadvantageously be implemented as silicon optical circuits having compactdimensions at relatively low cost. Silicon optical circuits employintegrated waveguide structures formed in a silicon layer of asilicon-on-insulator (SOI) substrate, forming a silicon photonic chip.

In some applications, the optical signal is injected in/extracted fromthe photonic chip in a near perpendicular fashion, with respect to thephotonic chip substrate plane, by way of an optical coupler (forexample, a grating coupler) formed in the silicon photonic chip forinput-output of the photonic signal. When using the silicon substrate insuch a coupling fashion, such as when coupling to an optical fiber, theoptical fiber is mounted in near perpendicular fashion.

During manufacture of integrated optical devices, a large number ofintegrated optical devices are fabricated on a typical semiconductorwafer. As part of a rigorous manufacturing process, it may be helpful tomeasure optical loss or spectral response for quality control at thewafer level. Since optical loss in individual optical components isrelatively low (for example, on the order of 0.015 dB per component),testing for loss is typically performed on a special purpose teststructure (i.e., the test structure will not be functional for thecustomer) comprising a plurality of optical components daisy chainedtogether to define a testing (reference) circuit. During testing, aknown/reference optical input is injected into the test structure andthe optical output is measured for comparison to the known/referenceoptical input. The overall loss of the testing circuit is determined anddivided by the number of test optical devices included therein todetermine an average device loss. If the determined loss is outside anacceptable range, the wafer is considered defective and removed from themanufacturing process.

One potential issue with testing individual devices at the wafer levelis that optical inputs and outputs must be precisely aligned toaccurately measure device loss. This alignment issue is worsened when anoptical fiber array needs to be aligned with multiple outputs (due tothe intrinsic misalignment within the array). This may cause the testingprocess to be quite long and difficult. Since this testing is done atthe wafer level before singulation, this laborious effort could growgeometrically if each wafer includes multiple testing circuits. Indeed,this task can be exhaustive when each IC within a wafer includes atesting circuit.

There is a need for a better method of testing and associated supportingtest apparatus.

SUMMARY

In an embodiment, a method is presented for testing an optical testingcircuit on a wafer, wherein said optical testing circuit includes: anoptical input configured to receive an optical test signal and aplurality of photodetectors configured to generate a correspondingplurality of electrical signals in response to optical processing ofsaid optical test signal through the optical testing circuit. The methodcomprises: applying the optical test signal with a sweep in wavelength;simultaneously sensing the plurality of electrical signals at each stepof the sweep in wavelength; generating test data for the simultaneouslysensed plurality of electrical signals at each step of the sweep inwavelength; storing the test data at each step of the sweep inwavelength; sensing a change in wavelength of the optical test signal;and in response to the sensed change in wavelength of the optical testsignal, outputting the stored test data.

In an embodiment, a system is presented for testing an optical testingcircuit on a wafer, wherein said optical testing circuit includes: anoptical input configured to receive an optical test signal and aplurality of photodetectors configured to generate a correspondingplurality of electrical signals in response to optical processing ofsaid optical test signal through the optical testing circuit. The systemcomprises: a light source configured to apply the optical test signalwith a sweep in wavelength; a probe card configured to simultaneouslysense the plurality of electrical signals at each step of the sweep inwavelength; a conversion circuit configured to generate test data forthe simultaneously sensed plurality of electrical signals at each stepof the sweep in wavelength and store the test data at each step of thesweep in wavelength; a power meter configured to sense a change inwavelength of the optical test signal; and a control circuit configuredto respond to the sensed change in wavelength of the optical test signalby reading the stored test data from the conversion circuit.

In an embodiment, a method is presented for testing an optical testingcircuit on a wafer, wherein said optical testing circuit includes: anoptical input configured to receive an optical test signal and aplurality of photodetectors configured to generate a correspondingplurality of electrical signals in response to optical processing ofsaid optical test signal through the optical testing circuit. The methodcomprises: applying the optical test signal with a sweep in wavelength;and at each step of the sweep in wavelength, performing a process asfollows: sequentially selecting an electrical signal from the pluralityof electrical signals; and for each selected electrical signal,performing a process as follows: sweeping the wavelength of the opticaltest signal; sensing the selected electrical signal; generating testdata from the selected electrical signal over the sweep in wavelength ofthe optical test signal; determining whether all electrical signals ofthe plurality of electrical signals have been selected; and if not, thenchanging the selected electrical signal and repeating the process.

In an embodiment, a system is provided for testing an optical testingcircuit on a wafer, wherein said optical testing circuit includes: anoptical input configured to receive an optical test signal and aplurality of photodetectors configured to generate a correspondingplurality of electrical signals in response to optical processing ofsaid optical test signal through the optical testing circuit. The systemcomprises: a light source configured to apply the optical test signal; aprobe card configured to simultaneously sense the plurality ofelectrical signals at each step of the sweep in wavelength; amultiplexer circuit configured to sequentially select each electricalsignal of the plurality of electrical signals; a meter circuitconfigured to convert each selected electrical signal to generate testdata for output; and a control circuit configured to control thesequential selection by the multiplexer circuit so that each electricalsignal of the plurality of electrical signals is selected and furthercontrol the light source to perform a sweep in wavelength of the opticaltest signal for each selected electrical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, by way of example only,with reference to the annexed drawings, in which:

FIG. 1 shows a semiconductor wafer;

FIG. 2 is a block diagram of a testing circuit;

FIGS. 3A-3D show examples of optical test circuits for inclusion withinthe testing circuit of FIG. 2;

FIGS. 4A-4D show examples of optical test circuits for inclusion withinthe testing circuit of FIG. 2;

FIG. 5 is a block diagram of an embodiment of a testing circuit;

FIG. 6 is a block diagram of an embodiment of a testing circuit;

FIG. 7 is a block diagram of an embodiment of a testing circuit;

FIG. 8 is a block diagram of an opto-electrical test measurement systemusing the reference circuit;

FIG. 9 is a block diagram of the signal acquisition system; and

FIG. 10 is a block diagram of the signal acquisition system.

DETAILED DESCRIPTION

The present disclosure will now be described more fully hereinafter withreference to the accompanying drawings, in which several embodiments ofthe invention are shown. This present disclosure may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the present disclosure to those skilled in theart.

FIG. 1 schematically shows a semiconductor wafer 34. The semiconductorwafer 34 may comprise, for example, a silicon wafer or silicon oninsulator (SOI) wafer, that includes a plurality of integrated circuitchips 33, fabricated using well known processes, therein. The chips 33are delimited from each other by scribe lines 37. Each chip 33 includesother circuitry 35, which represents functional circuitry for use aftersingulation.

A testing circuit 20 a is carried by the semiconductor wafer 34 withinthe scribe line 37. In this embodiment, the testing circuit 20 a isremoved/destroyed by the singulation process which dices thesemiconductor wafer 34 to individually release the plurality of chips 33included therein. In the illustrated embodiment, only one testingcircuit 20 a is shown in the scribe line 37 for the wafer. This is byexample only, as multiple testing circuits 20 a can be distributed overthe wafer 34 at various locations as needed or desired. In otherembodiments, a plurality of testing circuits 20 a may be provided in thescribe lines surrounding the circuitry 35 for one chip 33; for example,two reference circuits at opposing sides of a respective circuit 35 fora chip 33.

In other embodiments, a testing circuit 20 b may instead be locatedbeyond the plurality of scribe lines; i.e., the testing circuit 20 bremains within the chip 33, perhaps as part of the functional circuitry35, after singulation. In the illustrated embodiment, only one testingcircuit 20 b is shown within the chip 33. This is by example only, asmultiple testing circuits 20 b can be distributed within a single chip33 as needed or desired.

In some embodiments, the semiconductor wafer 34 can include one or moretest chips (TC) that comprise the testing circuit 20 b. Such test chipsTCs may include other testing circuitry 36 as well. In the illustratedembodiment, only one testing circuit 20 b is shown within the test chipTC. This is by example only, as multiple testing circuits 20 b can bedistributed within a single test chip TC as needed or desired.

A testing circuit 20 (encompassing either of the testing circuits 20 a,20 b) is used in the testing of the wafer 34. The number of testingcircuits 20 and their positioning on the wafer is dictated by the waferdesign and the type of testing to be performed. In situations where thattesting by the testing circuit 20 indicates presence of a defect, thewafer may be discarded prior to the singulation operation and thesubsequent packaging of the individual chips 33. Alternatively, theindividual chips 33 whose testing circuit 20 indicates presence of adefect can be discarded after singulation and sorting.

Reference is now made to FIG. 2 showing a block diagram of an embodimentfor the testing circuit 20. The testing circuit 20 receives an opticaltest signal 22 from an optical coupler 24 positioned on the wafer. Theoptical coupler 24 receives the optical test signal 22 from an opticalsource 26 (for example, a laser optical source) external to the wafer.The optical coupler 24 may, for example, comprise an optical gratingcoupler (for example, a single polarization grating coupler—SPGC). Theoptical test signal 22 is received at an input of an optical splitter28. In this implementation, the optical splitter 28 performs a 1×N splitof the optical test signal 22 to generate N optical test signals30(1)-30(N) for application to one or more included optical testcircuits 32.

Details of an example optical test circuit 32 are shown in FIG. 2. Theoptical test signal 30(1) is received at an input of a 1×2 balanced orcalibrated optical splitter 34. The optical splitter 34 performs a 1×2split of the optical test signal 30(1) to generate a testing opticaltest signal 36 and a reference optical test signal 38. The testingoptical test signal 36 is applied to a test channel 40 formed by aplurality of optical device under test (DUT) circuits (DUT 1) 42 a-42 ncoupled in a daisy chain configuration (i.e., in series) between theinput of the test channel 40 and the output of the test channel 40. Thereference optical test signal 38 is applied to the input of a referencechannel 44. The reference channel 44 is configured to not include DUTcircuits.

The DUT circuits 42 a-42 n are, for example, representative of, orperhaps replicas of, optical circuits forming at least part of thefunctional circuitry 35. In this regard, those skilled in the artunderstand that testing of the optical DUT circuits 42 in the testingchannel 40 will provide information concerning the operation of thecorresponding optical circuits of the functional circuitry 35; whereinfailure of the optical DUT circuits 42 in the test channel 40 wouldindicate a likelihood that the corresponding optical circuits of thefunctional circuitry are similarly flawed.

Each optical DUT circuit 42 may comprise, for example, one of an opticalwaveguide, or an optical modulator component, such as a meanderingoptical waveguide. The test channel 40 may be formed of optical DUTcircuits 42 a single respective device type; for example, the testchannel includes only optical waveguides in daisy chain connection.Alternatively, the test channel 40 may be formed of optical DUT circuits42 of different respective device types; for example, the test channelincludes an optical waveguide in daisy chain connection with an opticalmodulator. The embodiment shown by example in FIG. 2 shows that thattest channel 40 includes DUT circuits 42 of a single same type referredto as DUT 1.

The testing circuit 20 further includes a first photodetector (PD) 48coupled to receive an optical test output signal 50 from the output ofthe test channel 40 and a second photodetector 52 coupled to receive thereference optical test signal 38 output from the reference channel 44.The first and second photodetectors 48 and 52 may, for example, each beformed by a photodiode. The testing circuit 20 further includes a firstelectrical output terminal 56 coupled to the first photodetector 48 anda second electrical output terminal 58 coupled to the secondphotodetector 52. Electrical signals I₁ and I₂ are output from the firstand second electrical output terminals 56 and 58, respectively, withthose electrical signals being indicative of the optical test outputsignal 50 and the reference optical test signal 38 detected,respectively, by the photodetectors 48 and 52.

FIGS. 3A-3D show example configurations for other optical test circuits32 that could be used within the testing circuit 20 of FIG. 2. Thetesting circuit 20 may include one or more of each optical test circuit32 as needed to effectuate a desired optical testing protocol.

FIG. 3A shows an optical test circuit 32 with a test channel 40 a thatincludes a single optical DUT circuit 42 a that receives one of theoptical test signals 30(a) and outputs an optical test output signal 50a to a photodetector (PD) 48 a. An electrical output terminal 56 a iscoupled to the photodetector 48 a. An electrical signal I_(a) is outputfrom the electrical output terminal 56 a, with that electrical signalbeing indicative of the optical test output signal 50 a detected by thephotodetector 48 a.

The DUT circuit 42 a is, for example, representative of, or perhaps areplica of, an optical circuit forming at least part of the functionalcircuitry 35. In this regard, those skilled in the art understand thattesting of the optical DUT circuit 42 a in the testing channel 40 a willprovide information concerning the operation of the correspondingoptical circuits of the functional circuitry 35; wherein failure of theoptical DUT circuit 42 a in the testing channel 40 a would indicate alikelihood that the corresponding optical circuits of the functionalcircuitry are similarly flawed. The optical DUT circuit 42 a maycomprise, for example, one of an optical waveguide, or an opticalmodulator component, such as a meandering optical waveguide.

FIG. 3B shows an optical test circuit 32 with a test channel 40 b thatincludes a single optical DUT circuit 42 b that receives one of theoptical test signals 30(b) and outputs an optical test output signal 50b to a plurality of photodetectors (PD) 48 b-48 d. Correspondingelectrical output terminals 56 b-56 d are coupled to the photodetectors48 b-48 d. Electrical signals I_(b)-I_(d) are output from the electricaloutput terminals 56 b-56 d, with those electrical signals beingindicative of the optical test output signal 50 b detected by thephotodetectors 48 b-48 d.

The DUT circuit 42 b is, for example, representative of, or perhaps areplica of, an optical circuit forming at least part of the functionalcircuitry 35. In this regard, those skilled in the art understand thattesting of the optical DUT circuit 42 b in the testing channel 40 b willprovide information concerning the operation of the correspondingoptical circuits of the functional circuitry 35; wherein failure of theoptical DUT circuit 42 b in the testing channel 40 b would indicate alikelihood that the corresponding optical circuits of the functionalcircuitry are similarly flawed. The optical DUT circuit 42 b maycomprise, for example, one of an optical waveguide, or an opticalmodulator component, such as a meandering optical waveguide.

FIG. 3C shows an optical test circuit 32 with a test channel 40 c thatincludes a plurality of optical DUT circuits 42 c-42 n coupled in adaisy chain configuration (i.e., in series) between the input of thetest channel 40 c and the output of the test channel 40 c. The testchannel 40 c receives one of the optical test signals 30(c) and outputsan optical test output signal 50 c to a photodetector (PD) 48 c. Anelectrical output terminal 56 c is coupled to the photodetector 48 c. Anelectrical signal I_(c) is output from the electrical output terminal 56c, with that electrical signal being indicative of the optical testoutput signal 50 c detected by the photodetector 48 c.

The DUT circuits 42 c-42 n are, for example, representative of, orperhaps a replica of, optical circuits forming at least part of thefunctional circuitry 35. In this regard, those skilled in the artunderstand that testing of the optical DUT circuits 42 c-42 n in thetesting channel 40 c will provide information concerning the operationof the corresponding optical circuits of the functional circuitry 35;wherein failure of the optical DUT circuits 42 c-42 n in the testingchannel 40 c would indicate a likelihood that the corresponding opticalcircuits of the functional circuitry are similarly flawed. The opticalDUT circuits 42 c-42 n may each comprise, for example, an opticalwaveguide, or an optical modulator component, such as a meanderingoptical waveguide.

FIG. 3D shows an optical test circuit 32 with a test channel 40 d and areference channel 44 d. The optical test signal 30(d) is received at aninput of a 1×2 balanced or calibrated optical splitter 34 d. The opticalsplitter 34 d performs a 1×2 split of the optical test signal 30(d) togenerate a testing optical test signal 36 d for application to the testchannel 40 d and a reference optical test signal 38 d for application tothe reference channel 44 d. The test channel 40 d includes a pluralityof optical DUT circuits 42 d-42 n coupled in a daisy chain configuration(i.e., in series) between the input of the test channel 40 c and theoutput of the test channel 40 c. Here, it will be noted that a differenttype of DUT circuit, referred to as DUT 2, is being tested in this testchannel. The test channel 40 d receives the testing optical test signal36 d and outputs an optical test output signal 50 d to a photodetector(PD) 48 d. An electrical output terminal 56 d is coupled to thephotodetector 48 d. An electrical signal I_(1d) is output from theelectrical output terminal 56 d, with that electrical signal beingindicative of the optical test output signal 50 d detected by thephotodetector 48 d. A photodector (PD) 52 d receives the referenceoptical test signal 38 d. An electrical output terminal 58 d is coupledto the photodetector 52 d. An electrical signal I_(2d) is output fromthe electrical output terminal 58 d, with that electrical signal beingindicative of the reference optical test signal 38 d detected by thephotodetector 52 d.

The DUT circuits 42 d-42 n are, for example, representative of, orperhaps a replica of, optical circuits forming at least part of thefunctional circuitry 35. In this regard, those skilled in the artunderstand that testing of the optical DUT circuits 42 d-42 n in thetesting channel 40 d will provide information concerning the operationof the corresponding optical circuits of the functional circuitry 35;wherein failure of the optical DUT circuits 42 d-42 n in the testingchannel 40 d would indicate a likelihood that the corresponding opticalcircuits of the functional circuitry are similarly flawed. The opticalDUT circuits 42 d-42 n may each comprise, for example, an opticalwaveguide, or an optical modulator component, such as a meanderingoptical waveguide.

FIGS. 4A-4D show alternative embodiments for the optical test circuits32 shown in FIGS. 3A-3D. Like reference numbers refer to like or similarparts. The embodiments of FIGS. 4A-4D differ from the embodiments ofFIGS. 3A-3D by further including, within each test channel 40, a 1×2optical splitter 47 that splits the optical test output signals 50 intoa first optical signal 51 a and second optical signal 51 b. The firstoptical signal 51 a is detected by the corresponding photodetector 48,while the second optical signal 51 b is output from the optical testcircuit 32.

FIG. 5 shows a block diagram of an embodiment for the testing circuit 20wherein the optical test signal 22 received from the optical coupler 24is applied to an input of a 1×2 balanced or calibrated optical splitter60. The optical splitter 60 performs a 1×2 split of the optical testsignal 22 to generate a first optical test signal 22(1) and a secondoptical test signal 22(2). The first optical test signal 22(1) isapplied to a first optical splitter 62(1) which performs a 1×N split ofthe first optical test signal 22(1) to generate N optical test signals30(1)-30(N) for application to one or more included optical testcircuits 32. The test circuits 32 may, for example, comprises selectedones of the circuits shown in FIGS. 2, 3A-3D and 4A-4D. Electricalsignals from the photodetectors within the test circuits 32 are providedat corresponding electrical output terminals 70. The second optical testsignal 22(2) is applied to a second optical splitter 62(2) whichperforms a 1×N split of the second optical test signal 22(2) to generateN optical reference signals 38(1)-38(N) that are detected byphotodetectors 72. Electrical signals from photodetectors 72 areprovided at corresponding electrical output terminals 74.

Reference is now made to FIG. 6 which shows a block diagram of anembodiment for the testing circuit 20 wherein the optical test signal 22generated by the optical source 26 is split by a 1×2 balanced orcalibrated optical splitter 60′ to generate a first optical test signal22(1) and a second optical test signal 22(2). The first optical testsignal 22(1) is received by a first optical coupler 24(1) and applied toa first optical splitter 62(1) which performs a 1×N split of the firstoptical test signal 22(1) to generate N optical test signals 30(1)-30(N)for application to one or more included optical test circuits 32. Thetest circuits 32 may, for example, comprises selected ones of thecircuits shown in FIGS. 2, 3A-3D and 4A-4D. Electrical signals from thephotodetectors within the test circuits 32 are provided at correspondingelectrical output terminals 70. The second optical test signal 22(2) isreceived by a second optical coupler 24(2) and applied to a secondoptical splitter 62(2) which performs a 1×N split of the second opticaltest signal 22(2) to generate N optical reference signals 38(1)-38(N)that are detected by photodetectors 72. Electrical signals fromphotodetectors 72 are provided at corresponding electrical outputterminals 74.

As an alternative to use of a balanced optical splitter 60, the opticalsplitter may instead have a split ratio that is known (calibrated) basedon wavelength. Furthermore, as an alternative to use of a balancedoptical splitter 60, the optical splitter may instead comprise anoptical switch. The optical splitter 60′ in the FIG. 6 embodiment islocated off the wafer, while the optical splitter 60 in the FIG. 5embodiment is located on the wafer.

Reference is now made to FIG. 7 which shows a block diagram of anembodiment for the testing circuit 20. Like reference numbers refer tolike or similar parts. The embodiment of FIG. 7 differs from previouslydescribed embodiments in the shared use of photodetectors 48 within eachoptical test circuit 32 for processing both the optical test signals30(1)-30(N) and the optical reference signals 38(1)-38(N). In thisembodiment, an optical switch 60″ is used to split the optical testsignal 22 generated by the optical source 26 to generate a first opticaltest signal 22(1) and a second optical test signal 22(2). With use of anoptical switch, only one of the first optical test signal 22(1) orsecond optical test signal 22(2) is applied to the testing circuit 20 ata time, and thus the electrical signals output from the electricaloutput terminals 70 can be distinguished as relating to detection of theoptical test signals 30(1)-30(N) or detection of the optical referencesignals 38(1)-38(N).

Reference is now made to FIG. 8 showing an opto-electrical testmeasurement system 100 using the testing circuit 20. The testing circuit20 may comprise any one of the embodiments shown herein with referenceto FIGS. 2, 3A-3D, 4A-4D, 5, 6 and 7, fabricated on a wafer at anydesired location as described herein with reference to FIG. 1. Thetesting circuit includes an optical input 102 and a plurality ofelectrical outputs 104(1)-104(m) for electrical signals (such as currentsignals output by photodetectors). The optical input 102 comprises, forexample, one or more optical couplers as described herein. The pluralityof electrical outputs 104(1)-104(m) comprise, for example, theelectrical output terminals as described herein. In a preferredimplementation, the plurality of electrical outputs 104(1)-104(m) areimplemented by electrical pads on the wafer 34 that can be probed usingtest equipment well known to those skilled in the art to obtain theelectrical signals.

An optical source 106 (for example, a continuous wave laser lightsource) generates an optical signal 108. An optical splitter 110receives the optical signal 108 and splits the received optical signalinto an optical testing signal 112 and an optical reference signal 114.The optical testing signal 112 comprises the optical test signal 22 aspreviously described herein. The optical testing signal 112 is appliedto the optical input 102. The testing circuit 20 produces a plurality ofelectrical (i.e., photocurrent) signals at the plurality of electricaloutputs 104(1)-104(m) in response to processing of the optical testingsignal 112 through the reference circuit 20 in the manner describedherein in connection with FIGS. 2, 3A-3D, 4A-4D, 5, 6 and 7.

An electrical probe apparatus 120 (for example, in the form of a probecard well known to those skilled in the art for use in wafer probing fortest) is electrically coupled to the pads forming the plurality ofelectrical outputs 104(1)-104(m) to obtain the plurality of electrical(photocurrent) signals and provide a corresponding plurality of outputsignals 122.

A signal acquisition system 130 is coupled to receive the opticalreference signal 114 at an optical input and is further coupled toreceive the output signals 122 at a plurality of electrical inputs. Thesignal acquisition system 130 processes the optical reference signal 114and the output signals 122 to generate test data 132 for output to adata processing computer 134. The signal acquisition system 130 operatesto detect the wavelength of the reference signal 114, and measure thecurrent of the output signals 122 from the testing circuit 20. Thesignal acquisition system 130 may further operate to control the opticalsource 106.

The signal acquisition system 130 may be in further communication withthe computer 134 over a data bus 160 (for example, a Universal SerialBus (USB)). The computer 134 controls operation of the signalacquisition system 130 through the control information 158 and receivesdata output from the signal acquisition system 130. The controlinformation 158 is provided to instruct the signal acquisition system130 operation for setting the start wavelength and the stop wavelengthof the optical signal 108 generated by the optical source 106, theoptical power of the optical signal 108, the speed of a sweep of theoptical signal 108 between the start/stop wavelengths, and anidentification of the number of data points to capture over that sweep.The received data is stored by the computer 134 in a readable memory forlater analysis. The computer 134 may also directly compute the relevantdata in function of the device tested: for instance loss of devicecomputed from DUT and reference diode measurement.

Reference is now made to FIG. 9 showing a block diagram of the signalacquisition system 130. The signal acquisition system 130 receives theoptical reference signal 114 at the optical input and receives theoutput signals 122 at the plurality of electrical inputs. The pluralityof electrical inputs are coupled to inputs of a multiplexer circuit(MUX) 140 (or electromechanical switch circuit). The multiplexer circuit140 receives a control signal 142 that specifies a selection of one ofthe electrical inputs 122 for output as signal 144. A transimpedanceamplifier (TIA) 146 receives the signal 144 and outputs an amplifiedsignal 148 to a first input of a voltmeter 150. More particularly, theTIA 146 converts the current signal sensed from the electrical output104 and selected by the MUX 140 to a voltage V. The operationalparameters (gain, offset, etc.) of the TIA 146 are controlled by thecontrol information 158 provided by the data processing computer 134. Asecond input of the voltmeter 150 is coupled to the optical input toreceive the optical reference signal 114. The voltmeter 150 functions tomeasure the electrical voltages relating to the signal 114 at each stepof the sweep of the wavelength of the optical reference signal 114. Thevoltmeter 150 accordingly senses a change in wavelength of the opticalreference signal 114 during the sweep and makes the voltage measurementat each change in wavelength. The measured voltages are output to thedata processing computer 134 (FIG. 7) as the test data 132 at eachchange in wavelength. The control signals 142 are generated by a MUXcontroller 156 in response to the control information 158 provided bythe data processing computer 134. The MUX controller 156 functions tospecify the selection of one of the electrical inputs 122 for output assignal 144 response to data processing computer 134 instruction.

Operation of the opto-electrical test measurement system 100 using thesignal acquisition system 130 of FIG. 9 is as follows: a) the testequipment is coupled to the wafer and in particular to the referencecircuit 20 (this involves optically coupling the optical testing signal112 to the optical coupler 24 at the optical input 102, and electricallycoupling the probe card 120 to the plurality of electrical outputs104(1)-104(m); proper optical alignment is important); b) the controlsignal 142 output from the MUX controller 156 in response to signal 158controls the MUX 140 to select one of the output signals 122; c) thesignal 158 further adapts the gain of the TIA 146 in order ensure thatthe signal 148 for the selected output signal 122 remains within theoperating range of voltmeter 150; d) the computer 134 then controls theoptical source to sweep the wavelength of the optical signal 108 over adesired range; e) the voltmeter 150 measures the voltage of the selectedone of the output signals 122 at each step of the sweep in wavelength;f) when the sweep ends, the measured voltages for the selected one ofthe output signals 122 are output as the test data 132; g) adetermination is then made as to whether all of the output signals 122have been processed; h) if not, then the process returns to step b) forthe selection of a next one of the output signals 122 and the stepsc)-g) are repeated as needed until all of the output signals 122 havebeen processed.

Reference is now made to FIG. 10 showing a block diagram of analternative embodiment of the signal acquisition system 130. The signalacquisition system 130 of FIG. 10 includes a plurality of signalconversion circuits 170 equal in number to the number of photocurrentsignals of the output signals 122 produced by the probe card 120. Eachsignal conversion circuit 170 includes a logarithmic amplifier 172,coupled in series with a programmable gain amplifier 174, coupled inseries with an analog to digital converter (ADC) circuit 176, coupled inseries with a digital memory 178. The logarithmic amplifier 172 has aninput coupled to receive one of the photocurrent signals, and digitaldata corresponding to the amplified and converted photocurrent signal isstored in the memory 178.

A power meter 186 is coupled to the optical input to receive the opticalreference signal 114. The power meter 186 functions to capture thewavelength of the optical reference signal 114, sense a change inwavelength of the optical reference signal 114 during a sweep and asserta trigger signal 188 in response to each new wavelength. The power meter186 further outputs data on bus 160 to the computer 134 specifying eachwavelength of the optical reference signal 114 during the sweep.

The trigger signal 188 is applied to an input of a microcontroller 180.The microcontroller 180 further includes an input coupled to an outputof the digital memory 178 of each signal conversion circuit 170. Inresponse to assertion of the trigger signal 188, the microcontroller 180operates to read the digital data from the digital memory 178 of eachsignal conversion circuit 170 and generate the test data 132 at thecurrent optical signal wavelength for output to the computer 134. Themicrocontroller 180 further functions to command storage of the testdata 132 for each wavelength of the optical reference signal 114 duringthe sweep in response to assertion of the trigger signal 188. Themicrocontroller 180 also functions to generate control signaling 182applied to each signal conversion circuit 170 for the purpose ofcontrolling gain of the programmable gain amplifier 174 as well ascontrol timing of signal sampling/conversion operations performed by theADC circuit 176.

Operation of the opto-electrical test measurement system 100 using thesignal acquisition system 130 of FIG. 10 is as follows: a) the testequipment is coupled to the wafer and in particular to the referencecircuit 20 (this involves optically coupling the optical testing signal112 to the optical coupler 24 at the optical input 102, and electricallycoupling the probe card 120 to the plurality of electrical outputs104(1)-104(m); proper optical alignment is important); b) the opticalsource 106 operates to sweep the wavelength of the optical signal 108over a plurality of discrete wavelength steps; c) at each step of thewavelength sweep as detected by the power meter 186 through monitoringof change in wavelength of the optical reference signal 114, the triggersignal 188 is asserted; d) each signal conversion circuit 170 operates,in response to the control signaling 182, to convert the received outputsignal 122 to corresponding digital data stored in the memory 178 forthe current step in wavelength; e) the microcontroller 180 responds tothe asserted trigger signal 188 by reading (for example, in parallel)the digital data from the signal conversion circuits 170 and outputting(for example, in serial) the read digital data for that wavelength tothe computer 134; f) the power meter 186 sends data comprising thewavelength of the monitored optical reference signal 114 for the currentstep to the computer 134; and g) the process ends when the opticalsource 106 completes its sweep of the wavelength of the optical signal108 and data for each discrete step in the wavelength sweep has beencollected and sent to the computer 134.

Although the implementation of FIG. 10 uses a logarithmic amplifier 172coupled in series with a programmable gain amplifier 174 to analogprocess the analog photocurrent signals of the output signals 122, itwill be understood that this is just example of the analog signalprocessing that is performed. Alternatively, other types ofamplification and analog signal processing, such as through the use of atransimpedance amplifier, that are compatible with subsequent ADCprocessing could be used.

A programmable processor, such as a digital signal processor (DSP) 190,could be provided to post process the read digital data and generate thetest data.

Many modifications and other embodiments of the present disclosure willcome to the mind of one skilled in the art having the benefit of theteachings presented in the foregoing descriptions and the associateddrawings. Therefore, it is understood that the present disclosure is notto be limited to the specific embodiments disclosed, and thatmodifications and embodiments are intended to be included within thescope of the appended claims.

The invention claimed is:
 1. An apparatus for testing an optical testingcircuit on a wafer, wherein said optical testing circuit includes: anoptical input configured to receive an optical test signal and aplurality of photodetectors configured to generate a correspondingplurality of electrical signals in response to optical processing ofsaid optical test signal through the optical testing circuit,comprising: a probe card including a plurality of electrical probesconfigured to probe the wafer and receive the plurality of electricalsignals; a multiplexer circuit coupled to the probe card to receive theplurality of electrical signals and configured to sequentially select anelectrical signal from the plurality of electrical signals generated bythe plurality of photodetectors for output as a selected electricalsignal; and a testing circuit configured, for each selected electricalsignal, to: sweep the wavelength of the optical test signal over a rangeof wavelengths from a start wavelength to a stop wavelength; generatetest data from the selected electrical signal over the sweep inwavelength of the optical test signal; determine whether all electricalsignals of the plurality of electrical signals have been selected; andif not, then control the multiplexer to change the selected electricalsignal and repeat.
 2. A testing circuit, comprising: an optical circuitconfigured to apply an optical test signal to an optical input with asweep in wavelength including a plurality of steps over a range ofwavelengths from a start wavelength to a stop wavelength, wherein asignal derived from said optical test signal is passed through at leastone optical device under test (DUT) circuit; a photodetector circuitconfigured to detect the optical test signal and a signal output fromthe DUT circuit to generate a corresponding plurality of electricalsignals; and a circuit operating, for each step of the sweep inwavelength, to: simultaneously receive the plurality of electricalsignals; generate test data from the simultaneously received pluralityof electrical signals; store the test data; detect a change in the stepof the sweep in wavelength; and output the stored test data in responseto the detected change in step.
 3. The testing circuit of claim 2,wherein each electrical signal of said plurality of electrical signalsis converted to a digital signal to generate the test data.
 4. Thetesting circuit of claim 3, further comprising a memory configured tostore the digital signal for the test data.
 5. The testing circuit ofclaim 4, wherein the circuit reads the stored digital signal from thememory.
 6. The testing circuit of claim 2, further comprisingcontrolling a gain of each electrical signal of the plurality ofelectrical signals at each step of the sweep in wavelength, whereincontrolling the gain comprises adapting gain so that the electricalsignal is within an operating range.
 7. The testing circuit of claim 2,wherein the output stored test data includes an identification of thestep in the sweep in wavelength of the optical test signal thatcorresponds to the simultaneously received plurality of electricalsignals from which the stored test data is generated.
 8. The testingcircuit of claim 7, wherein the identification of the step is anidentification of the wavelength for that step.